Injection of Prolactin (PRL) into fetal rabbits caused in our hand increase in lung phospholipid contents. Injection of PRL had no effect on the pressure volume relation of fetal lung. Injection of bromocryptin into pregnant rabbits caused a significant reduction in fetal lung phospholipid. Injection of estrogene had no effect. Injection of bromocryptin, estrogene, levodopa and methyldopa into pregnant rats had no effect on phospholipid content of fetal rat lungs. In the human, a high correlation was found between gestational age and umbilical cord PRL levels, the PRL levels in cord blood of infants suffering from Respiratory Distress Syndrome (RDS) were significantly lower than in healthy infants. A similar relationship was found between T4 and RDS. T4 and PRL correlated in the infants with RDS, but not in healthy infants. We conclude at this stage, that lung maturation is under multiple hormonal control. The interaction of these hormones is not yet understood, but our findings and the findings of others suggest a hypothalamic trigger for lung maturation.